Traditionally, gases containing a malodorous component, harmful component, organic solvent, hydrocarbon vapor or the like (hereinafter referred to as the subject gas) have been treated by a number of methods, including the direct combustion method, in which the subject gas is oxidized at a high temperature of 600.degree. to 1000.degree. C., the catalytic oxidation method, in which the subject gas is oxidized at relatively low temperature in the presence of catalyst, the adsorption method, which uses an adsorbent such as activated charcoal, silica gel, activated alumina, activated clay or zeolite, and the absorption method, in which an absorbent solution is used to physically or chemically absorb and eliminate malodorous and other undesirable components.
However, the direct combustion method requires a lot of fuel and increases running cost when the combustible substance concentration in the subject gas is low. The catalytic oxidation method also requires a lot of energy for heating to raise a temperature of the entire subject gas when the oxidizable substance concentration in the subject gas is low. For these reasons, there has recently been carried out the method in which the malodorous and combustible substance is once adsorbed to adsorbent and then desorbed with a small amount of heating gas to take it out as a concentrated combustible gas, which is then oxidatively decomposed in a separately installed catalytic reaction apparatus. In this case, however, cost is high and a large area of installing space is required because two apparatuses are necessary, namely an apparatus for concentration by adsorption-desorption and a catalytic reaction apparatus. As for the adsorption method and the absorption method, either of them is unsatisfactory because the deactivated adsorbent or absorbent solution must be regenerated using a separate apparatus or disposed.
On the other hand, there have recently been investigated the direct ozonic oxidation method, in which ozone is used to oxidize malodorous components etc. in vapor phase at room temperature or at a temperature lower than that used for the conventional catalytic oxidation method, and the ozonic catalytic oxidation method, in which ozone and a catalyst are used in combination. These ozonic oxidation methods draw much attention as methods for treating the subject gas because they have many advantages; for example, the desired effect is obtained with only a trace amount of ozone, the starting material is air and is easy to supply, and a fungicidal effect is expected in addition to the oxidizing effect. However, even the ozonic catalytic oxidation method, which is appreciated as a method with excellent oxidative decomposition performance, can undergo degradation of its oxidative decomposition performance due to accumulation of a high boiling, difficult-to-decompose compound such as phenol or a decomposition product thereof when treating them at room temperature. Another problem is that even in the case of a compound which is treatable at room temperature, catalyst exchange or regeneration must be made in long term use since the oxidative decomposition performance declines due to accumulation of trace amounts of undecomposed components, though the oxidative decomposition performance can be kept intact in short term use.